Overlay system with digital optical transmitter for digitized narrowcast signals

Abstract

Methods and apparatuses are provided to modify existing overlay system architectures in a cost effective manner to meet the growing demand for narrowcast services and to position the existing overlay systems for additional future modifications. The implementations of the improved overlay system of this disclosure re-digitize narrowcast analog signals after they have been QAM modulated and upconverted to RF frequencies and replace the analog narrowcast transmitters with digital narrowcast transmitters. In the fiber nodes, the received narrowcast signals are converted back to analog signals and combined with analog broadcast signals for transmission to the service groups.

The invention claimed is: 
     
       1. A transmitting headend or transmitting node in an overlay transmission system for transmitting broadcast and narrowcast services, the transmitting headed or transmitting node in the overlay transmission system comprising:
 an analog optical transmitter and a plurality of narrowcast digital optical transmitters overlaid for transmissions to a receiving node, the analog optical transmitter extending in a downstream direction toward the receiving node via a first fiber and the plurality of narrowcast digital optical transmitters extending in a downstream direction toward the receiving node via a second fiber; 
 the analog optical transmitter configured for converting a received analog signal to a broadcast radio frequency (RF) optically modulated signal for transmission on the first fiber; 
 a plurality of digitizers each configured to:
 receive an analog signal derived from one or more digital bit streams that each have been QAM encoded and modulated; and 
 for transmission to the plurality of narrowcast digital optical transmitters, re-digitize the analog signal derived from the one or more digital bit streams, 
 
 wherein re-digitizing by the plurality of digitizers produces a plurality of digital bit streams for transmission by the plurality of narrowcast digital optical transmitters to the receiving node for maintaining independence of an optical level of the re-digitized signal received from the respective digitizer; 
 the plurality of narrowcast digital optical transmitters configured to receive the plurality of digital bit streams from the plurality of digitizers, respectively, and configured to convert each of the plurality of digital bit streams to a digital narrowcast signal optically modulated at a respective wavelength for digital transmission on the second fiber; and 
 a optical multiplexer configured to multiplex the plurality of narrowcast optically modulated signals to produce a multi-wavelength digitally modulated optical signal for the digital transmission on the second fiber. 
 
     
     
       2. The overlay system of  claim 1  wherein the multiplexer comprises a dense wavelength division multiplexer. 
     
     
       3. A method of digitizing a forward path in an overlay system, the method comprising:
 receiving at least one analog signal for conversion to a broadcast signal for transmission to a node; 
 converting a received analog signal to a broadcast radio frequency (RF) optically modulated signal for transmission on a first fiber; 
 receiving a plurality of analog signals wherein each analog signal is derived from one or more digital bit streams that have been QAM encoded and modulated; 
 re-digitizing each of the plurality of analog signals derived from the one or more digital bit streams that have been QAM encoded and modulated to produce corresponding re-digitized signals, wherein re-digitizing each of the plurality of analog signals produces a plurality of digital bit streams for transmission by a plurality of narrowcast digital optical transmitters to a receiving node for maintaining independence of an optical level of the re-digitized signals; 
 converting each of the plurality of digital bit streams to a digital narrowcast signal optically modulated at a respective wavelength for digital transmission on the second fiber; and 
 multiplexing the plurality of narrowcast optically modulated signals to produce a multi-wavelength digitally modulated optical signal for the digital transmission on the second fiber to the node. 
 
     
     
       4. The method of  claim 3  wherein multiplexing the plurality of optical signals comprises dense wavelength division multiplexing the plurality of optical signals.